A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via projection using a projection system onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
There is a continuing desire to be able to generate patterns with finer resolution. In general, shorter wavelength radiation may be used in order to achieve a finer resolution pattern. Step and scan systems are becoming resolution limited, particularly using radiation of 193 nm wavelength. Resolution has been extended using immersion lithography which allows a numerical aperture (NA) increase to approximately 1.56 NA. This will support 32 nm (half pitch) resolution. To go to higher resolution, particularly using 193 nm wavelength illumination, will require the development of new patterning techniques.
A patterning technique that has been proposed to increase the resolution is the multiple exposure technique. In this technique, target portions on the substrate are exposed twice or more. Such multiple exposures can be done using a different pattern for each exposure or using a different optical setting in the projection system or the illumination system of the lithographic apparatus or both. In case a different pattern is used for the different exposures, these different patterns can, e.g., be provided by different patterning devices. As an example, it may be desirable for a substrate to have both an exposure using a phase shift mask and an exposure with a trim mask. It may be desirable that both exposures have different exposure conditions. Conventionally, such a ‘double exposure’ is obtained by first exposing the entire substrate with a first patterning device (e.g., a phase shift mask), then exchanging the first patterning device with a second patterning device (e.g., a trim mask) and exposing the entire substrate with the second patterning device. This procedure is rather time consuming and generally results in inferior performance with respect to throughput (i.e., number of substrates that is processed per unit of time).
A possible drawback of changing the patterning devices can be mitigated by using multiple patterning devices on one stage, as described in U.S. Pat. No. 6,800,408. Despite the use of multiple patterning devices on one stage, the patterning technique of U.S. Pat. No. 6,800,408 may still have a significant impact on the throughput of the apparatus compared to a single exposure technique. This is due to the consecutive patterning of the first and the second patterning devices onto the target portion of the substrate. This drawback may be mitigated by simultaneously patterning the first and second patterning devices onto the target portion of the substrate, as described in U.S. Pat. No. 6,611,316. U.S. Pat. No. 6,611,316 describes a patterning technique wherein two reticle images are produced side-by-side in a field plane of the projection system.